This invention concerns vacuum breakers, and more particularly the improvement of stationary and moving electrodes of vacuum breakers.
Vacuum breakers are disclosed for example in Japanese Patent Publication No. 8499/1975 and Japanese Patent Laid-Open No. 56323/1985. FIG. 11 is a profile section showing the structure of a conventional electromagnetic driving vacuum breaker as described in Patent Publication No. 8499/1975. In this figure, 101 is a vacuum switch tube. It comprises a container under vacuum consisting of an insulator housing 7, a fixed end plate 6a and moving end plate 6b, and bellows 3. The rod-shaped stationary conductor 2a traverses the fixed end plate 6a, its joints being of such a construction as to maintain the vacuum inside the container. The moving conductor 2b opposite stationary conductor 2a traverses bellows cover 4 fitted to the end of bellows 3, the joint between moving conductor 2a and bellows cover 4 also being so constructed as to maintain the internal vacuum. At the ends of stationary conductor 2a and moving conductor 2b which are facing each other, a stationary electrode 1a and moving electrode 1b are installed. The other end of stationary conductor 2a is secured to a fixed terminal 102. The moving conductor 2b is driven in direction A by a control mechanism 103 via a hinge 104. The moving shunt 105 is a flexible conductor, one end being connected to moving conductor 2b and the other end to fixed terminal 106.
In the above vacuum breaker, current flows via fixed terminal 106, moving shunt 105, moving conductor 2b, moving electrode 1b, fixed electrode 1a, fixed conductor 2a and fixed terminal 102.
We shall explain the closing action of the vacuum breaker with reference to FIGS. 10A, B and C. FIG. 10A shows the stroke characteristic of moving conductor 2b. At time t.sub.0, the control mechanism 103 begins operating, and exerts a force which pushes the moving conductor 2b toward the upper part of FIG. 11. As the bellows 3 are free to extend and contract, moving conductor 2b moves upwards. At time t.sub.2, stationary electrode 1a and moving electrode 1b come into contact. After these electrodes have chattered several times (3 times in FIG. 12A), they touch each other finally. FIG. 12B shows the closing action of the electrodes set up by this chattering. Chattering occurs, moreover, whether the switch has an electrical load or not. The chattering frequency and the total chattering period vary depending on the roughness of the electrodes, and the speed of motion of moving electrode 2b driven by control mechanism 103.
In general, at the time of closing, a making current produces an electromagnetic repulsion between the electrodes so that the electrodes remain apart longer during chattering. Also, in case of closing high voltages, a pre-arc is set up at a time t.sub.1 before the time t.sub.2 at which metal contact actually occurs when the distance 1 between the electrodes is less than a specified value. As shown in FIG. 12C, therefore, when the breaker is closed for passing making current under a high voltage, there is a pre-arc time P, and arcing times T.sub.1, T.sub.2, T.sub.3 when the electrodes are opened due to chattering. Melting due to the heat of the arc and generation of heat due to metal contact are repeated several times during closing action. The sum of the shaded areas in FIG. 12C (arcing times) (current x time) is related to the heat of the arc produced, and the arc heat accounts for most of the energy input to the electrodes. As arc heat increases, electrode melting and wear become very obvious, and the temperature rises. At the same time there is increased deposition on the electrodes. This deposition sometimes makes it impossible to separate the electrodes. This kind of serious trouble is often mainly due to excessive arc heat.
The main performance characteristics of vacuum breakers, namely breaking performance, deposition property, wear resistance, breakdown voltage and current chopping performance depend largely on the material of the electrodes. In general, however, these characteristics are contradictory to each other. For example, electrode materials which are excellent for breaking give unsatisfactory deposition property. In conventional vacuum breakers, materials with excellent circuit breaking properties were used even though their use did result in poorer deposition performance. To prevent accidents due to deposition, however, it was necessary to supply high energies to control mechanism 103 so as to increase the external pressure on the electrodes and increase the force pulling them apart. As a result, the control meachanism not only had to be bulky and costly, but the life of the bellows and fixed end plate was shortened due to mechanical fatigue under the increased external pressure. Various means were devised in an attempt to overcome these disadvantages. In the device shown in FIG. 11, the direction of the current flowing in shunt 105 is reversed in the V-shaped section, and the electromagnetic repulsion produced in this section was used to apply an upward pressure to moving conductor 2b.
As shunt 105 is installed at some distance away from conductor 2b, however, some time delay is required for the applied pressure to be transmitted to the conductor. This device was therefore not necessarily effective in preventing chattering or preventing the electrode from floating up. Various designs for terminal 102 were attempted in order to restrict chattering, but as the chattering depends on the roughness of the electrodes, it was found to be extremely difficult to suppress it to a stable level throughout the entire life of the breaker.